1. Field of the Invention
This invention relates to a new process for the preparation of urethane group-containing polyisocyanates which contain only a small proportion of monomeric starting diisocyanates and to the products of this process.
2. Description of the Prior Art
Aromatic polyisocyanates containing urethane groups, especially those based on diisocyanatotoluene, are valuable "lacquer polyisocyanates." They are described, for example, in "Lehrbuch der Lacke und Beschichtungen" by H. Kittel, publishers W. A. Colomb, Berlin 1973, Volume I, Part 2. They are generally prepared by the reaction of low molecular weight, polyhydric alcohols with an excess of diisocyanates followed by removal of the excess diisocyanate by distillation or extraction.
In the process of extraction, the excess diisocyanate can be carefully removed by means of suitable solvent mixtures. One important disadvantage of this method is that it requires the use of large quantities of extracting agent which is difficult to subsequently separate, recover and work up such that the process is in most cases uneconomical on a technical scale.
Moreover, extraction removes not only the monomeric diisocyanates but also a proportion of low molecular weight polyisocyanates from the polyisocyanate mixture, thereby altering the properties of the polymer.
Removal of the excess diisocyanate by distillation is most effectively carried out by the method of thin layer distillation but since a polyisocyanate which is free from monomers has a very high softening point and a high fusion viscosity, this distillation must be carried out at temperatures from 160.degree. to 180.degree. C.; however,decomposition and polymerization reactions occur at these temperatures. This in turn leads to the deposition of polymers and severe mechanical strain on the thin layer distillation apparatus.
A simplified method of separating monomers from low molecular weight aromatic polyisocyanates containing urethane groups is described in DE-PS 214,847, in which the polyisocyanates containing urethane groups are modified with aromatic polyisocyanates containing acyl urea, allophanate or biuret structures. After removal of the excess monomeric diisocyanates by extraction or distillation, the polyisocyanate obtained has a low viscosity and high isocyanate content. One major disadvantage of these products, however, is their high content of monomeric diisocyanate, which may be as much as 0.6 to 1.95%, based on solids. This high proportion of free starting diisocyanate severely limits the use of these products in numerous fields for reasons of workplace hygiene.
It has now been found that polyisocyanates containing urethane groups based on aromatic diisocyanates, in particular on diisocyanatotoluene, may be obtained with a low viscosity, a high isocyanate content and a proportion of free starting diisocyanate of not more than 0.4%, preferably 0.3% by weight if the distillative removal of excess monomeric starting diisocyanate is carried out in the presence of aliphatic polyisocyanates containing isocyanurate groups and the product obtained as distillation residue is after-treated, if necessary, with a sub-equivalent quantity of monohydric and/or polyhydric alcohols in the presence of an inert solvent.